The invention relates to the field of hydraulic pilot control arrangements for piloting a spool valve.
Spool valves are for example used for controlling the flow and direction of hydraulic fluid sent to an actuator. In some cases, for example when a spool valve is used in a high pressure power hydraulic circuit, its spool is not directly controlled mechanically or electrically, but it is preferably piloted hydraulically through a hydraulic pilot circuit. In a hydraulically piloted spool valve, the movements of the spool are due to the application of a pilot hydraulic pressure on the spool, thereby modifying the position of the spool with respect to a body of the valve and thereby changing the flow of the high pressure hydraulic fluid through the valve. Generally, the pilot pressure is lower than the high pressure of the power circuit. In some cases, the hydraulic pilot circuit comprises a first and a second pilot hydraulic lines connected to opposite sides of the spool of the spool valve, and comprises pilot pressure regulating means for selectively establishing or removing a pilot pressure in each of said first and second pressure lines. In most cases, it is provided that pilot pressure can be sent to only one side of the spool at a time. The spool valve can be biased towards a central position where it returns when no pilot pressure in sent on either side of the spool. The pilot pressure regulating means are for example in the form of joystick-type hydraulic controller.
One known problem of such type of pilot control arrangements is that, when pilot pressure is established in one pilot line by the pressure regulating means, the pressure establishment in that line can be quite sudden, which results in a sudden movement of the spool, which will in turn into a quite sudden application of pressure to the power actuator fed through the spool valve. This results in a quite brutal starting movement of the actuator, which is in most cases not desirable. The same effects can be seen when pilot pressure is removed from a corresponding pilot line: sudden movement of the spool, and therefore sudden stop of actuator are inevitable.
To limit the sudden stop and start movements of the actuator, it is known to arrange flow limiters in the hydraulic pilot lines for limiting the flow of hydraulic fluid from the spool to the pilot pressure regulating means and inversely. Those flow limiters therefore slow down the movement of the spool when it leaves or goes back to its central position, and thereby dampen the stopping movement. Flow limiters can be arranged as fixed throttles in the pilot lines. But, in some cases, these fixed throttles need to be of a very small diameter to be effective, and they then present the risk of getting suddenly clogged by some impurities contained in the hydraulic fluid. Upon starting, such clogging would simply translate in the actuator not moving, which is already a problem. But when it comes to stopping, the clogging would translate in the actuator not stopping its movement which can have very severe consequences.
Therefore, there is a need to design a new hydraulic pilot pressure arrangement which may dampen the start and stop movements of the actuator, without having the risks of clogging known with the currently used damping technologies.
The invention relates, according to an aspect thereof, to a hydraulic pilot control arrangement for a spool valve, wherein the spool of the spool valve is biased towards a central position, wherein the arrangement comprises a first and a second pilot hydraulic line connected to opposite sides of the spool of the spool valve and wherein pilot pressure regulating means are provided for selectively establishing or removing a pilot pressure in each of said first and second pressure lines, characterized in that the arrangement comprises a damping device including a chamber divided into a first and a second pressure compartment by a sliding piston, the first and second pressure compartments being connected respectively to the first and second pilot hydraulic lines, and said piston being biased towards a rest position in the chamber.